The present invention relates to prosthetic vascular structures and, more particularly, to vascular prostheses fabricated from highly expanded polytetrafluoroethylene.
Frequently in cardiovascular surgery, it is necessary to bypass or replace blood vessels, whether veins or arteries, to assure an adequate and balanced blood flow to particular organs, extremities or areas of the body.
Unsuccessful attempts were made during the early years of this century to implant prosthetic or artificial vessels fabricated from glass and metal. With the availability of inert synthetic materials such as nylon, Orlon, Dacron and Teflon (polytetrafluoroethylene or PTFE) during the late 1940""s and early 1950""s, large arterial replacements were achieved with increasing degrees of success.
Cardiovascular surgeons presently have available knitted and woven vessels of Dacron and Teflon which may be used as replacements for arteries having relatively large inside diameters (approximately 7 millimeters). However, no clinically acceptable small arterial prosthesis has been available; and surgeons have found it necessary to scavenge marginally important or superficial vessels, such as the saphenous vein, to serve as replacements for defective small bore arteries.
It is a principal objective of the present invention to provide a prosthetic vascular structure capable of replacing or bypassing natural blood vessels having relatively small inside diameters as well as those vessels of intermediate and large bore.
The transplantation of saphenous veins from the patient""s legs to more critical portions of the cardiovascular system entails numerous disadvantages: The entire surgical procedure is unduly protracted by having to first excise the venous replacement from one part of the patient""s body, then prepare the replacement for implantation, and finally implant the substitute vessel at another point in the patient""s cardiovascular system. Prolonged exposure to anesthesia and multiple incisions combine to increase the probability of both infection and post-operative discomfort.
Cardiovascular surgery frequently requires grafts of various lengths and diameters in achieving, for example: the femoral artery to popliteal artery bypass, the coronary artery bypass, the renal artery bypass, etc.. Occasionally, however, especially in older patients, the saphenous veins themselves are inadequate for use as replacements; and in some instances only unacceptably short segments of the saphenous veins are available for transplantation.
It is another object of the present invention to provide an artificial vascular structure which may be prefabricated in various lengths and diameters, thereby eliminating unnecessary incisions, minimizing exposure to anesthesia, conserving already limited surgical resources, and assuring an ample supply of small bore vascular replacements.
Operations such as the femoral/popliteal bypass require especially long grafts which ideally taper in cross-sectional area from their proximal to their distal ends. Heretofore, transplanted saphenous veins have been used to accomplish this bypass procedure. Because blood flow through the saphenous vein is unidirectional in character, it is necessary to reverse the vein when it is being used as an arterial substitute. The inside diameter of a saphenous vein naturally tapers between its proximal and distal ends. Reversing the vein for implantation between the femoral and popliteal arteries results in a corresponding reversal of this taper so that the smaller diameter end must be grafted to the relatively large femoral artery while the larger diameter end is grafted to the relatively small popliteal artery. The reversed taper of an implanted saphenous vein causes deceleration of the blood flow while the turbulence inducing discontinuities at the bypass junctures contribute to stasis and associated thrombosis.
It is a further object of the present invention to provide a small bore prosthetic vascular structure which may be fabricated in relatively long segments, which segments decrease in inside diameter between proximal and distal ends so as to facilitate their implantation as peripheral artery replacements and assure a close hemodynamic simulation of the corresponding natural vessel.
The inner surface of natural blood vessels is characterized by a thin, delicate layer of endothelial cells known as the intima. The primary function of this layer is to provide a smooth interface between the blood stream and the vessel wall. For example, a ruptured artery may, after healing, include rough or irregular protrusions from the wall into the blood stream. As the natural intima reestablishes itself over the wound area, it serves to lessen the severity Ln irregular wall transitions and thereby assure laminar blood flow.
While the outer surface of a vessel prosthesis is encapsulated by fibrous growth as a result of normal rejection processes, the inner surface typically becomes isolated from the blood stream by a layer which has been referred to, with varying degrees of accuracy, as the neointima or pseudointima. To be classified as a true neointima, it would be necessary for the inner surface of the artificial vessel to be covered with an extremely thin lining of viable endothelial cells. Although such a lining would vary in thickness from point to point, it would be typically less than ten cells thick. There has heretofore existed no known vessel prosthesis of any size or configuration which, when implanted, would support the growth and maintenance of a true neointima layer. Accordingly, the vessel/blood interface associated with state of the art prostheses is characterized by a pseudointima consisting at best of a few irregularly distributed islands of endothelial growth but made up largely of compacted fibrin which has been flow sculptured by the blood stream. Occasionally, portions of this pseudointima will fracture or particulate and introduce emboli into the patient""s blood stream.
The formation and maintenance of a true neointima requires a continuous extravascular source of nourishment to supplement whatever nourishment might be supplied by diffusion from the adjacent blood stream. In extremely short grafts (less than 2 to 3 centimeters), cellular ingrowth has been observed along the inside surfaces from the suture lines at the ends of the graft. In such cases, the tissue growth, augmented by capillary ingrowth, can provide a continuous nutrition route capable of supporting a viable neointima. However, the thickness of this inner layer of tissue is so great as to virtually occlude all but large bore grafts. Not only is ingrowth of this type unpredictable but it can be expected or tolerated at all only in very short grafts of relatively large inside diameter.
Extensive efforts have been made toward the fabrication of a porous vascular structure which would permit uniform transmural tissue ingrowth sufficient to assure the formation and continuous nutrition of a true neointima layer. The culmination of this prior art effort is represented by grafts which are machine woven from threads consisting of tightly twisted Dacron or Teflon fibers. The threads used in the manufacture of these woven grafts, while extremely small by garment industry standards, are enormous when viewed in the context of a hemodynamic system under pressure. As is true of any knitted or women fabric, the minimum size of the interstices between threads is determined by the diameter of the threads themselves. Because these interstices or voids are quite large in woven grafts, it is necessary to preclot the graft by dipping it in blood in order to prevent excessive transmural leakage after implantation.
The large size of the threads used in prior art woven grafts and the tightness with which the constituent fibers are twisted renders each individual thread virtually impenetrable to cellular ingrowth and virtually beyond cellular circummigration. From the viewpoint of a single fibroblast, a knitted vessel prosthesis looks like two or three mammoth structures separated by equally mammoth voids which have been filled by the clotting process with masses of coagulated fibrin and proteinaceous matter. Viewed on a similar microscopic basis, the inner wall of the knitted graft appears as a series of large, rough cylinders of inert material separated by cavities which are equal or larger in breadth and depth to the diameter of the cylinders. As the blood flows through a prosthetic vessel of this type, the cavities are filled with slow moving blood while the irregularity and protrusion of the threads promotes turbulent blood flow. Thrombosis throughout the wall of the graft and at its inner surface, combined with the large size and separation of the knitted threads, serves to initiate buildup of an irregular pseudointima layer while at the same time blocking and inhibiting transmural cellular ingrowth of the type necessary for the support of a uniform, viable neointima.
Accordingly, it is a major objective of the present invention to provide a homogeneously porous vascular prosthesis characterized by small nodes interconnected by extremely fine fibrils to form an open superstructure which will allow uniform, controlled transmural cellular ingrowth and thereby assure the establishment and maintenance of a thin, viable neointima as well as firm structural integration of the graft into the body.
It is another objective of the present invention to provide a porous vascular prosthesis characterized by a superstructure which is substantially impermeable to the flow of relatively high viscosity liquids such as blood at normal pressures.
Briefly stated, the invention constitutes a prosthetic vascular device formed from a small bore tube of polytetrafluoroethylene which has been heated, expanded and sintered so as to have a microscopic superstructure of uniformly distributed nodes interconnected by fibrils and characterized by: (a) an average internodular distance which is (i) large enough to allow transmural migration of typical red cells and fibroblast, and (ii) small enough to inhibit both transmural blood flow at normal pressures and excessive tissue ingrowth; and (b) an average wall thickness which is (i) small enough to provide proper mechanical conformity to adjacent cardiovascular structures, and (ii) large enough, when taken in conjunction with the associated internodular distance, to prevent leakage and excess tissue ingrowth, to allow free and uniform transmilral nutrient flow, and to assure mechanical strength and ease of implantation.